Frequency shift offset quadrature modulation and demodulation

Abstract

A Frequency Shift Offset Quadrature (FSOQ) signal which is a constant envelope frequency-shift keyed version of an offset quaternary phase shift keyed (O-QPSK) transmission is provided. A technique for generating an FSOQ signal in a manner similar to O-QPSK by modulating offset quadrature channels with selectable pulse shapes which yields the desired 3-tone continuous phase FSK approximation of O-QPSK is disclosed. Receiver synchronization circuits are also provided which enable FSOQ to be detected as a special case of O-QPSK, whereby the detection performance of synchronous coherent QPSK, rather than the relatively inferior performance of FSK detection, is obtained. FSOQ provides a constant envelope and requires less bandwidth than minimum shift keying (MSK).

I claim: 
     
       1. A method of modulating a signal for transmission comprising: generating first and second binary bit sequences related to first and second data sequences respectively, said first and second binary bit sequences being offset in time with respect to each other;   selectively generating one of (i) a sinusoidal signal, and (ii) a rectangular signal, for selected portions of said first and second binary bit sequences, to produce respective third and fourth modulation sequences, the polarities of said sinusoidal and rectangular signals selected as a function of the polarities of said first and second binary bit sequences, said one of said sinusoidal and rectangular signals being selected as a function of bit transitions of said first and second binary sequences; and   modulating a carrier signal with said third and fourth modulation sequences to provide a transmission signal.   
     
     
       2. The method of claim 1 wherein said transmission signal is frequency modulated and has a substantially constant envelope. 
     
     
       3. The method of claims 1 or 2 wherein the steps of selectively generating comprise: generating first and second sinusoidal signal sequences as a function of said first and second binary bit sequences, respectively;   generating first and second rectangular signal sequences as a function of said first and second binary bit sequences, respectively:   detecting bit transitions of said first and second binary bit sequences to provide a gating signal; and   selectively modulating said carrier with one of (i) said first and second rectangular signal sequences and (ii) said first and second sinusoidal signal sequences in accordance with said gating signal.   
     
     
       4. The method of claim 3 wherein the step of detecting bit transitions includes detecting the absence of bit transitions over a preselected time period in both of said first and second binary bit sequences to produce a first gate signal, and detecting the presence of at least one bit transition over said preselected time period in at least one of said first and second binary bit sequences to produce a second gate signal. 
     
     
       5. The method of claim 4 wherein said carrier is modulated (i) with said first and second rectangular sequences in response to said first gate signal, and (ii) with said first and second sinusoidal sequences in response to said second gate signal. 
     
     
       6. A method of demodulating a received signal modulated with a plurality of pulse shapes representing a bit pattern comprising: combining said received signal with a synthesized carrier signal to provide a baseband signal;   filtering said baseband signal to provide a filtered signal;   sampling said filtered signal in accordance with a signal synchronized with said bit pattern to detect the values of said bit pattern; and   optimizing said filtering of said baseband signal for the pulse shape having the least energy of said plurality of pulse shapes.   
     
     
       7. The method of claim 6 wherein said step of combining is performed in phase quadrature to produce in-phase and quadrature baseband signals, and said steps of filtering and sampling are provided for both of said in-phase and quadrature signals. 
     
     
       8. An apparatus for modulating a signal for transmission comprising: means for generating first and second binary bit sequences related to first and second data sequences, respectively, said first and second binary bit sequences being offset in time with respect to each other;   means for selectively generating one of (i) a sinusoidal signal, and (ii) a rectangular signal, for selected portions of said first and second binary bit sequences to produce respective third and fourth modulation sequences, the polarities of said sinusoidal and rectangular signals being selected as a function of the polarities of said first and second binary bit sequences, said means for selectively generating selecting said one of said sinusoidal and rectangular signals as a function of bit transitions of said first and second binary sequences; and   modulating means for modulating a carrier signal with said third and fourth modulation sequences to provide a transmission signal.   
     
     
       9. The apparatus of claim 8 wherein said means for selectively generating comprises: means for generating first and second sinusoidal sequences in response to said first and second binary bit sequences, respectively;   means for generating first and second rectangular signal sequences in response to said first and second binary bit sequences, respectively;   means for detecting bit transitions of said first and second binary bit sequences to provide a gating signal; and   means for selectively delivering to said modulating means one of (i) said first and second rectangular signal sequences and (ii) said first and second sinusoidal signal sequences in accordance with said gating signal.   
     
     
       10. The apparatus of claim 9 wherein said means for detecting bit transitions detects the absence of bit transitions over a preselected period of time in both of said first and second binary bit sequences to produce a first gate signal, and further detects the presence of at least one bit transition over said preselected period of time in at least one of said first and second binary bit sequences to produce a second gate signal. 
     
     
       11. The apparatus of claim 10 wherein said means for selectively generating further comprises first gate means receiving said first gate signal, said first gate means passing said first and second rectangular signal sequences to said modulating means in response to said first gate signal; and second gate means receiving said second gate signal for passing said first and second sinusoidal signal sequences to said modulating means in response to said second gate signal.   
     
     
       12. The apparatus of claim 11 wherein said modulating means comprises: first mixer means receiving at one input the selected one of (i) said first rectangular and (ii) said first sinusoidal signal sequences;   second mixer means receiving at one input the selected one of (i) said second rectangular and (ii) said second sinusoidal signal sequences;   said first and second mixers receiving at another input a carrier signal in phase quadrature; and   signal combining means for combining output signals from said first and second mixer means to produce a signal for transmission.   
     
     
       13. A method of synchronizing a receiver with an incoming signal having a carrier reference signal modulated in accordance with a bit pattern to thereby provide said incoming signal with components having frequencies above, below and at the frequency of said carrier reference signal, said method comprising: removing the modulation on said reference signal in accordance with a first function to provide a first reference signal;   mixing said first reference signal with signals related to said components having frequencies above and below the frequency of said carrier reference signal to provide first and second mixed signals, respectively; and   combining said first and second mixed signals to provide a combined signal synchronized with said bit pattern.   
     
     
       14. The method of claim 13 further comprising: detecting the zero crossings of said combined signal to thereby provide a first sync signal in accordance with positive-going zero crossings, and a second sync signal in accordance with negative-going zero crossings. 
     
     
       15. The method of claims 13 or 14 further comprising transforming said first reference signal in accordance with an inverse of said first function to provide a coherent carrier reference. 
     
     
       16. The method of claim 15 wherein said first function includes a frequency doubling. 
     
     
       17. An apparatus for synchronizing a receiver with an incoming signal having a carrier reference signal modulated in accordance with a bit pattern to thereby provide said incoming signal with components having frequencies above, below, and at the frequency of said carrier reference signal, said method comprising: means for removing the modulation on said carrier reference signal in accordance with a first function to provide a first reference signal;   means for mixing said first reference signal with signals related to said components having frequencies above and below the frequency of said carrier reference signal to provide first and second mixed signals, respectively; and   means for combining said first and second mixed signals to provide a combined signal synchronized with said bit pattern.   
     
     
       18. The apparatus of claim 17 further comprising zero crossing detector means for detecting zero crossings of said combined signal to thereby provide a first sync signal in accordance with positive-going zero crossings and a second sync signal in accordance with negative-going zero crossings. 
     
     
       19. The apparatus of claims 17 or 18 further comprising means for transforming said first reference signal in accordance with an inverse of said first function to provide a coherent carrier reference. 
     
     
       20. The apparatus of claim 19 wherein said first function includes a frequency doubling.